1. Field of the Invention
The present invention relates to an electrophotographic system image forming apparatus.
2. Description of the Related Art
In an electrophotographic system image forming apparatus, a charging amount of toner is easily influenced by an environment condition, that is, a surrounding ambient temperature and humidity or the like. For example, when the humidity is changed, an amount of moisture attached to the surface of developer is changed. Therefore, the charging amount of toner is changed, and thus the density of an output image is varied.
Accordingly, control has been suggested in which an absolute humidity (amount of moisture per unit volume) or relative humidity in developer is detected from a result detected by an environment sensor installed inside an apparatus body or near a development device and an image formation condition is changed (see Japanese Patent Laid-Open No. 2006-139140).
In humidity conditioning of developer, some time is required and it takes more time to perform moisture absorption than moisture removal. The humidity conditioning refers to a phenomenon in which a previously formed ambient state is adapted to a currently formed ambient state. The moisture absorption refers to humidity conditioning from low humidity to high humidity. The moisture removal refers to humidity conditioning from high humidity to low humidity.
Thus, according to Japanese Patent No. 2808108, developer humidity conditioning characteristics are corrected by history control. Specifically, the control is performed such that a change in temperature or humidity is measured by an environment sensor for each predetermined time, a humidity conditioning direction and a humidity conditioning degree are determined, and a development contrast potential is calculated.
Further, according to Japanese Patent Laid-Open No. 2007-41233, control is performed such that an idle rotation time of a developing container (developer) is determined based on a pause time of an image forming apparatus to correct a change in the charging amount of toner based on the pause time.
However, with recent improvement in color of a document, a high demand for stability of a density or a color has been made. In order to satisfy the demand, it is important to accurately comprehend and control a behavior of a humidity change of the developer. However, the demand has not been satisfied in the related art.
For example, in Japanese Patent Laid-Open No. 2006-139140 and Japanese Patent No. 2808108, during the pause time of an image forming apparatus, a humidity conditioning state distribution (hereinafter, referred to as uneven humidity conditioning) between the developer coming into contact with the surrounding ambient and the developer of a lower portion of a development device coming into no contact with the ambient is not considered on a sleeve and the surface of the developer. Therefore, in Japanese Patent Laid-Open No. 2006-139140 and Japanese Patent No. 2808108, it is difficult to perform the control with high accuracy, when an image forming operation is paused.
On the other hand, in Japanese Patent Laid-Open No. 2007-41233, the uneven humidity conditioning is resolved by performing idle rotation during the pause time and stirring the developer. However, since the idle rotation is performed excessively, a problem may arise in that a transfer failure of image due to deterioration in toner (specifically, an external additive is buried to toner) may occur or a process start time of an image forming apparatus body may increase.